Interpretive Summary: Distiller’s dried grains with solubles (DDGS), a by-product of the ethanol industry, is relatively inexpensive, rich in protein, fat, vitamins B-complexes and the immunostimulant B-glucan, and does not contain antinutritional factors commonly found in plant protein sources. Its use in aquaculture diets, however, is very limited due to lack of information on its nutritional value for various fish species. Thus, this study was conducted to evaluate the influence of the dietary levels of DDGS on growth performances, immune responses and resistance of Nile tilapia to Streptococcus iniae challenge. Results show that DDGS can be incorporated in Nile tilapia diets at a level of 20% as a replacement of the soybean meal-corn meal combination without affecting the growth, feed and protein efficiency, survival, and whole body composition. A level of 40% DDGS can be used if the diet is supplemented with crystalline lysine. Dietary levels of DDGS up to 40% in diets, with or without lysine supplementation, had no effect on immune responses and the resistance of tilapia to Streptococcus iniae infection.
Information generated from this study may allow flexibility in least-cost feed formulation, a reduction in feed costs, an increase in profits for producers, as well as increase the market demand for DDGS.

Technical Abstract:
This study was conducted to evaluate the effect of dietary levels of distiller’s dried grains with solubles (DDGS) on growth performance, body composition, hematology, immune response and resistance of Nile tilapia to Streptococcus iniae challenge. Five isocaloric diets containing DDGS at levels of 0, 10, 20 and 40%, and 40% DDGS + lysine (diet 1 to 5) as partial replacements of a combination of soybean meal (SBM) and corn meal on an equal protein basis were fed to juvenile Nile tilapia (9.41 ± 0.14 g) in four replicate aquaria twice daily to apparent satiation for 10 weeks. Results showed that of fish fed diet 4 had significantly lower (P<0.05) weight gain than those fed diets 1, 2 and 3, but did not differ from fish fed diet 5. Weight gain of fish fed diet 5 was significantly lower than that of fish fed diet 2 but was not different from those fed other diets. No significant differences were found in feed consumption and survival of fish in various treatments. Feed efficiency of diet 4 was significantly lower than that of diet 1 but was not different from those of other diets. Protein efficiency ratio (PER) of diet 4 was significantly lower than those of diets 1, 2 and 3 but did not differ from that of diet 5. PER of diet 5 was significantly lower than that of diet 1 but did not differ for those of other diets. No significant differences were observed among whole body moisture, lipid and ash. Fish fed diet 4 had significantly lower body protein than the group fed diet 1 but did not differ from those fed other diets. Red and white blood cell counts and hematocrit did not differ among treatments. Plasma protein, lysozyme and post-challenge agglutinating antibody titers against S. iniae were not affected by dietary treatments. Likewise, there were no significant differences among the average number of days to first mortality after S. iniae challenge and cumulative mortality 14 days post-challenge among fish in various treatments. DDGS can be incorporated in tilapia diet at a level of 20% as a substitute for a combination of SBM and corn meal without affecting their growth performance, body composition, hematological parameters, immune response and resistance to S. iniae infection.